In 1989 approximately 700 million pounds of polyethylene terephthalate (PET) were consumed to produce soft drink bottles. Presently about 28% of this material is recycled with the remainder being placed in landfills or incinerated. Due to problems with overloaded landfills and the negative environmental image plastics has received recently, bills have been introduced aimed at establishing bottle deposits and the number of curbside recycling programs has increased tremendously. The collection of PET bottles through these programs has resulted in a source of post-consumer PET which has been used for a number of applications. Generally these applications involve the conversion and fabrication of the post-consumer PET into materials of lower value. Examples of such applications are polyols for unsaturated polyesters or polyurethanes, fiberfill, carpet fibers, and strapping. Recycled PET is also blended with other materials such as polybutylene terephthalate, polycarbonate, or glass fibers, etc., for automotive as well as other engineering applications.
Post-consumer PET can also be recycled into resin which can be used in manufacturing containers for foods and beverages, such as carbonated beverage bottles. In such a procedure, the post-consumer PET is depolymerized to oligomers which are subsequently utilized as a raw material in the preparation of the recycled PET resin. Such a procedure for depolymerizing PET is described in U.S. Pat. No. 3,703,488.
It is, of course, necessary to separate the post-consumer PET from other plastics in the recycling procedure. For instance, polyvinyl chloride (PVC) is often present in sources of post-consumer PET. PVC presents a particular problem in that it is a clear plastic material which is sometimes misidentified as PET and in conventional processes the separation of PVC from PET is difficult, if not impossible. In any case, sources of post-consumer PET are often contaminated with PVC.
PET articles which are being recycled are typically ground into flakes for further processing. For instance, PET beverage bottles are typically ground into flakes which have a cross-sectional area of from about 4 mm.sup.2 to about 100 mm.sup.2. It is more typical for such flakes to have an area of about 15 mm.sup.2 to about 65 mm.sup.2. The thickness of the flakes varies with the wall thickness of the bottles which are being recycled.
Because post-consumer PET sources are often contaminated with PVC, recycle streams of PET flakes also generally contain PVC flakes. It is important to remove these PVC flakes from the PET flakes in the recycle stream to allow for further steps in the recycling operation. However, the removal of the PVC flakes from the PET flakes in the recycle stream has not proven to be straight forward. For instance, PVC generally has a density which is close to the density of PET. This renders flotation separation techniques which are based upon density differences of the flakes virtually impossible.
U.S. Pat. No. 4,064,677 discloses a technique for separating various plastics by an aqueous flotation procedure. In the procedure described in U.S. Pat. No. 4,064,677, the less wettable plastic floats to the surface of the aqueous medium by virtue of the fact that a greater number of gas bubbles adhere to its surface. However, PET and PVC have contact angles which are relatively close to one another. This problem is compounded by the fact that the contact angle of PVC can vary considerably based upon the manner in which it was compounded. Because the contact angle of compounded PVC is close to the contact angle of PET, separation based upon a differential density and wettability is extremely difficult without conditioning the PVC and PET in accordance with this invention.